The present invention relates to a transparent electrostatic image transfer recording sheet. More specifically, the present invention relates to a transparent recording sheet which permits more complete image transfer and fusing of toner into the novel image layer of the recording sheet.
In recent years, color copying machines and color laser printers employing an electrostatic image transfer system have been developed. According to this system, printing is conducted in such a manner that an image is optically formed on a transfer roller, and a toner composed of colorant carrying resin particles is electrostatically adsorbed on the latent image, and the adsorbed toner is transferred to an image receiving recording sheet, followed by fixing of the image.
Advances in electrophotography have resulted in the introduction of a new generation of color laser printers and copiers. The Cannon CLC-500 copier and Tektronix Phaser 540 printer represent some of the many new entrants. Most of the applications for electrophotography are related to paper based hard copies. Paper has an intrinsic volume conductivity and a sufficient pores volume to work well in color laser devices.
However, a large portion of the hard copies made with color laser printers and copiers are also done to produce transparencies useful for overhead projectors, i.e., OHP transparencies. Such OHP transparencies are used to make presentation slides, and color slides have been found to be replacing black and white copies.
The transparency generally involves a transparent resin sheet such as a polyester sheet, e.g., polyethylene terephthalate. The fixing of the image to the transparency, however, can cause problems since it involves fusing. The image is generally fixed and the temperature range is from 140 to 195 degrees C. which requires a great deal of thermal stability on the part of the OHP transparency composite. The thermal fixing also often involves pressing, and therefore occurs at considerable pressures which may cause serious deformations in the film transparency. Problems are often observed when commercially available OHP transparencies are used to make the transparent electrophotographic images. For example, when the thermal mechanical stability of any element of a OHP transparency is poor, distortion of the film occurs in the fuser and material will not exit from the printer easily. While the transparencies of today are made of many different plastic films other then the polyesters, such as polycarbonates and cellulose derivatives, all of them are subject to triboelectrical charging. When this charge occurs in the feed tray of the printer or a copier during a single film advancing motion, the sheet behind that first copy becomes electrostatically attracted to the first one, and moves together with that first sheet. This undesirable movement is called double pick-up or a mispick, which can seriously effect the transport reliability of the material in the system working in the unattended mode, i.e., in the absence of an operator.
In the fusing station there is an application of silicon oil which is needed to prevent an image transfer from the OHP transparency onto the fixing roller. Most of the commercial OHP transparencies show a large amount of silicon oil on the surface of the finished copy.
As discussed above, polyester films are often used as a carrier in today's OHP transparencies. The imaging layer of most commercially available transparencies consist of either acrylic of fully esterified epoxy resins, often mixed with quaternary ammonium ionically conductive polymers. Such systems generally have a glass transition point of from 55 to 75 degrees C. A back side coating is almost invariably an acrylic resin, which contains polymeric quarternized ionic conductors and spacer particles formed by large 5 to 10 microns polymeric beads, made from urea-formaldehyde or acrylic resins.
It has been found that such ionically conductive compounds have a tendency to migrate to the surface and create a condition where the surface resistivity drops below 10.sup.10 ohms/sq which causes an incomplete charging of the backside due to the fast charge dissipation. This phenomenon causes toner dropouts, which result in image defects. Quaternary polymers can also aggregate during the migration process and interfere with light transmission as well as with completeness of the fusing process. All of these effects result in an incomplete image transfer, poor fusion of the toner and sharp changes in refractive indexes along any direction in the imaged areas. During projection, these defects are seen as dark bands (incomplete fusing) or white spots (incomplete toner transfer).
Commercial designs of the OHP transparencies have also been found to exhibit many other undesirable deficiencies. For example, commercial designs are generally incapable of producing an image with a relatively low haze value. Another disadvantage of existing commercial materials is a propensity to absorb significant amounts of the silicon oil applied during the fixing process, which can also result in poor imaging as the oil interferes with the fusion process. When an incomplete or poor fusion occurs, the toner particles are not connected and there is a lot of light scattering from the edges of the individual toners, resulting in light escaping the collimating lens of the projector and showing muddy color with poor image definition.
There is a need in the industry therefore to provide an OHP transparency which forms highly defined good quality projectable images. A projected image is good when the haze of the image is low and sharp images are projected. Furthermore, it is important that reliable transport properties are exhibited by the OHP transparency in the printer. The OHP transparency is reliable when only a single sheet is transported during an individual imaging cycle. The conventional approach utilizing the above-mentioned components of the imaging layer and components of the chargeable layer do not satisfy these requirements.
Accordingly, it is an object of the present invention to provide an OHP transparency for color laser printers and copiers which overcome all of the aforediscussed deficiencies.
In another embodiment of the present invention, there is provided a novel overhead transparency which permits complete transfer of the toner with complete fusion of the toner.
In yet another object of the present invention, there is provided an overhead transparency which exhibits reliable behavior in the printer such that a single sheet is transported at any one time.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.